Engine oil leaks

[chuck459]

The Lycoming IO-360-A3BCD in my M20J is plagued with oil leaks.  When one is fixed, another develops.  Now there are several.  The leaks are coming from the base of cylinders and from a suspected crack in the case.    My shop has been unable to stop them and as a last resort recommends an engine overhaul.  My engine has 1,300 hours since the last overhaul but runs perfectly with good compression, equal EGT's, slightly high but stable oil consumption and not a trace of metal in the oil filter.  Penn Yan wants $55K for an exchange engine.  Someone has told me that my oil pressure could be too high.  Where is oil pressure measured and can it be adjusted?  Any comments or advise would be appreciated.

[Slick Nick]

Yes oil pressure can be adjusted, what is your oil pressure currently?

[Bartman]

Leaks under the cylinder are often due to the short segment of oil return hose connecting the line from the cylinder head to the block. You can replace the hose segments fairly easily. 

[Bartman]

I had a leak that I thought was one of the bolts going all the way thru the case. It ended up being a cracked case. If it’s not the oil return hose segments, you may find what I did. 

[Fritz1]

If not done so already have shop tighten clamps on oil return line hoses, wash engine down with mineral spirits and spray suspect area with dye penetrant developer, run engine and look for developing oil taces, may have to do this several times fixing one leak after the other, find out in which areas your case is prone to cracking, spray those areas with white dye penetrant developer, cracks will show up as dark lines in the developer, this is time consuming but beats tearing down a healthy engine

[cliffy]

Oil pressure is not your problem  

Engine oil leaks don't DRIP from the pressure side of the oil system. 

Oil leaks are not that hard to find usually

There are not that many places leaks can occur.

Leaks can come from the rocker box drain lines  that have the rubber hose connections.

Have these ever been replaced?

They are not hard to do and just tightening the clamps will not always stop the leaks if they are old-replace them

Leaks at the base of the cylinder (s) is another matter as is a cracked case. 

A cracked case is a grounding issue 

"Suspected" cracked case is not a viable action to determine the need for a new engine

It HAS to be verified that it  is cracked not just suspected. 

Have the bolts holding the bottom oil sump been checked for torque? Is the leak at that parting surface?

Lots of things to check before a new engine is called for. 

Your shop needs to wash the engine completely and then check it for leaks after a short flight. 

Finding leaks is not rocket science Verifying where they are coming from and fixing them is not that hard for a decent shop. 

Verify where they are coming from and fix them  It may take a few flights.

It can be any accessory on the rear of the engine leaking, tach drive seal, prop gov leaking, but they can be found.

The BIG issues would be cylinder base leaks and a VERIFIED cracked case leak.

Cylinder base leaks can be repaired with the top overhaul  NOT a full overhaul (it would be the cylinder base oring leaking)

A c racked crankcase is another matter requiring full overhaul.

AN "Old school" trick from 50 years ago was if the leak was at a parting surface and the bolts were correctly torqued and if it

still leaked- 5 min epoxy was spread on the parting surface and the leak went away :-)  Just "old school" operations NOT  FAA approved. 

How many years since overhaul? Ever had a top overhaul? 

You have a lot of items to check and fix before you spend the money for a new engine

 

 

 

[TN N246BB]

I just recently bought a Mooney M20J, had a pretty decent oil leak. We had a pretty hard time pinpointing finally figured out. It was the front main seal, it was not the shaft part that was leaking. It was the outside part of the seal, which therefore did not throw oil everywhere it is soaked the underside of the engine and made a big oil mess everywhere! Just got done replacing it and washing down the engine with degreaser and it runs beautiful nice and dry.

[Pinecone]

UV dye is great to find leaks.  Wash the engine with mineral spirits.  Add a small amount of dye into oil.  Run for 2 minutes.  In darkened area (hangar is fine) go over with UV light.  Oil leaks glow.

 

[Gee Bee Aeroproducts]

We have silicone version of

69603 hose with lined clamps

FluroSilicone hose version of std2180 hose with lined clamps

Ams3320.093 silicone with fiberglass valve cover gaskets 

 

let me know if you want better spec material then

milh6000 hose

cork vc 

[KSMooniac]

Dang, I just replaced my Lyc STD-2180 hoses and I wish I knew you had an improved kit!  

[Gee Bee Aeroproducts]

The key is lined clamps and Flurosilicone hose

mil hose mil h 6000 lasts about fives years

 

its a very old spec 

Probably from the 1940"s

Only Rotax has updated their spec 

[Flyler]

On 12/27/2025 at 10:50 PM, TN N246BB said:

I just recently bought a Mooney M20J, had a pretty decent oil leak. We had a pretty hard time pinpointing finally figured out. It was the front main seal, it was not the shaft part that was leaking. It was the outside part of the seal, which therefore did not throw oil everywhere it is soaked the underside of the engine and made a big oil mess everywhere! Just got done replacing it and washing down the engine with degreaser and it runs beautiful nice and dry.

I just bought a 201 recently and it went straight in for pre-buy and annual.  Shop found a leaking crank seal so they replaced it.  Well, they didn't do the best job. It sort of "blew out" and I had to replace it with local (and great) A&P.

The seal we removed appeared to have a tiny bit of sealant on it, only in two short areas.  Perhaps the rest was blasted out with the oil.

New airplane stuff!

[Yetti]

crank seal can be checked by putting a finger on it and turning the prop.  See if it moves.    Make sure the engine is off.